# Convergent extreme reductive evolution in ancient planthopper symbioses

## TL;DR

Ancient bacterial symbionts of planthoppers have evolved the smallest known genomes, revealing extreme genome reduction and loss of biosynthetic functions.

## Contribution

The study reveals convergent evolution of the smallest non-organellar bacterial genomes in planthopper symbionts.

## Key findings

- Vidania genomes in planthoppers are as small as 50-52 kb, the smallest non-organellar bacterial genomes known.
- These genomes have converged in gene content, retaining only the ability to produce phenylalanine for the host.
- The extreme reduction in cell-function genes places Vidania near the bacteria-organelle boundary in host dependence.

## Abstract

Strictly heritable endosymbiotic bacteria that provide limiting nutrients to sap-sucking hemipteran insects are known for their highly reduced genomes conserved in organization and function. Here, we show how in ancestral endosymbionts of planthoppers, Sulcia and Vidania, which have been gradually losing genes during ~263 my of co-diversification with hosts, co-infections by additional microbes and host ecological switches coincided with more dramatic genomic changes. At its extremes, this has resulted in the smallest non-organellar bacterial genomes known, at barely 50-52 kb. Such minuscule Vidania genomes evolved convergently in two planthopper superfamilies, and are strikingly similar in gene contents, including the ability to produce a single amino acid (phenylalanine) for the host. Losing many additional cell-function genes places them very close to organelles of symbiotic origin in the level of host dependence, further blurring the bacteria-organelle boundary.

Symbiotic bacteria can have exceedingly small genomes. This study finds that ancient bacterial symbionts of planthoppers have repeatedly evolved the smallest known genomes, losing most biosynthetic functions, revealing how extreme genome reduction shapes life at the edge of cellular complexity.